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Aircraft Engine Overhaul and Repair Basicsby Jim Cavanagh & Brian Lindquist | May 5, 2021

Turbo Prop Engine Maintenance
Turbo Prop Engine Maintenance

We look at the basics of Aircraft Engine Overhaul specifically Piston and Turbine Engines, identifying leading OEM Engines, approaches by engine type, hot section, borescope inspection, time limits and hourly cost maintenance.

Aircraft engines, like aircraft themselves, wear out. While engine reliability has progressed to the point where engines can spend a decade or more ‘on wing,’ all engines will – at some point – need to undergo an overhaul (or “major event”). The original equipment manufacturers (OEMs) of engines used in numerous small and large cabin aircraft have created very specific means of expediting engine overhaul safely. Below we look at Piston and Turbine Engines focused on the Business Jet and General Aviation Marketplace.

Most piston engines such as those produced by Continental and Lycoming have a limited time between overhaul (TBO), usually in the order of 1,200 – 2,200 hours, and often accompanied by a calendar time limit (e.g. 12 years). There are scores of smaller part 145 (FAA Approved) repair shops that can overhaul these engines, and many maintenance tasks can also be performed by a local A&P Mechanic. As with most engine maintenance, repair and overhaul (MRO) tasks, owners will have the choice of using factory new parts or third-party parts (aka PMA). While PMA parts will often be cheaper, they may void the manufacturer’s warranty, thereby leaving operators unprotected in the event of future failures.

When an engine is sent back to the OEM for overhaul, it may be rebuilt with parts which have been sourced from other engines and refurbished “as new” by the OEM to its rigorous specifications. This use of ‘part pooling’ is a common and efficient way of expediting the overhaul process, and also enables the OEM to avoid any supply chain issues affecting new parts supply. For example, new Continental engines were for a limited period built using overhauled lifters when the company had trouble with a vendor.

Some diesel engines, such as the Continental CD-155 powering the Cessna Turbo Skyhawk JT-A, come not with a TBO but a TBR: time between replacement. Operators of these aircraft replace their engines with a brand-new engine upon reaching 2,100 hours (or 12 years) of operation, rather than having their powerplant overhauled.

Turbine engines are another breed. Highly sophisticated, these engines require dedicated tooling, highly skilled and certified technicians, and infrastructures (e.g. testbeds) that can handle the associated power ratings. Many Part 145 repair stations can perform limited on-wing services for turbine engines such as borescope inspections and hot-section inspections (HSIs), but lack the dedicated design and engineering departments that an OEM-authorized engine MRO can offer.

Turbine engines have components that run on time limits as well as cycle limits. When a turbine engine requires overhaul, these are the factors that make someone realize the complexity of the engine itself. There are precise measurements made, blade balancing, and clearances that have to be met to make these engines perform at their greatest potential. Any inefficiencies or mistakes can have a drastic effect on performance, or make an engine not run at all. Components come at a high cost, and this is why most engines are put in a test cell before being installed on the aircraft. Many engines fail multiple times in a test cell, and this shows the importance of quality by the OEM repair facility. 

Overhaul approaches for turboprops and turbofans vary from engine to engine. Many mature powerplants, such as the ubiquitous Pratt & Whitney Canada PT6A, employ a basic TBO with a ‘hard-time’ (e.g. 3,600 hours in the case of most PT6A variants). By comparison, most newer engines such as Honeywell’s HTF7000 can be operated on an “on-condition” basis, facilitated through the use of engine health monitoring (EHM) systems.

When it comes to Jet Engine Maintenance, operators are usually supported through well-established networks of OEM-authorized overhaul centers (with the unique exception of FJ33/FJ44 manufacturer Williams International, which performs all major maintenance in-house). Operators typically arrange for their local FBO to perform engine removal, the compact size of turbine powerplants allowing them to be easily crated and shipped to the selected overhaul location. If needs be, an operator may choose to install a rental engine to enable continued aircraft operation until the original engine is returned and reinstalled.

Most engine OEMs also offer ‘pay per hour’ programs, which allow operators to enjoy ‘worry-free’ operation of their powerplants. These programs, which include GE Aviation’s OnPoint, Honeywell’s Maintenance Service Plan (MSP), P&WC’s Eagle Service Plan (ESP), Rolls-Royce’s Corporate Care and Williams International’s TAP Blue, as well as independent offerings such as JSSI’s hourly cost maintenance program and StandardAero’s VMAX offering, can provide full coverage for an engine, including normal maintenance, inspections, foreign object damage (FOD) repair, corrosion repair, and service bulletin compliance.

Many MRO providers offer a range of additional services to assist operators, including mobile service teams (MSTs) which are able to assist with in-the-field tasks including engine removal and installation. MROs are also usually able to source spares engines for operators, either to help cover for an engine which is undergoing overhaul, or to assist with aircraft-on-the-ground (AOG) events which cannot be addressed in the field.

The important thing about engines is that they need to be flying. Down time does not generate income. Planning ahead of time for upcoming engine tasks saves time and money in the long-run and creates peace of mind for any owner or maintenance manager.

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